This invention relates to ultrasonic Doppler flowmeters for the measurement of flow of a fluid containing reflectors and more particularly flowmeters having the capability of distinguishing between energy reflected from flowing reflectors and energy reflected from vibrating reflectors under zero-flow condition.
In ultrasonic Dopper flowmeters, an oscillator having a frequency f.sub.o is connected to apply this frequency to an ultrasonic transducer so that a beam of ultrasonic waves is propagated in a flowing fluid at an angle .theta. with respect to the direction of the flow. Part of the ultrasonic energy is reflected back to the transducer by air bubbles or particles in the fluid. If all of the reflectors were traveling at the same velocity as the fluid, the frequency of the reflected energy would be shifted from the transmitted frequency f.sub.o by an amount F.sub.d by virtue of the Doppler effect. The quantity F.sub.d is given by the well known equation EQU F.sub.d =2V.sub.f f.sub.o (cos .theta./C) (1)
where V.sub.f is the fluid velocity and C is the acoustic velocity in the fluid.
In practice, however, the received signal is not a single frequency but a broad spectrum of frequencies. This spectrum is produced because the particles do not all move at the same velocity, as each particle has a velocity which depends on its radial position in the pipe, and furthermore, the transmitted and received acoustic waves are not plane waves, but exhibit curved phase fronts. The resulting frequency spectrum is usually roughly Gaussian in shape with a half-power width equal to a mean frequency F.sub.d.
Another problem that is usually encountered is that the reflectors in the fluid have imparted to them vibrational velocity components in addition to the desired flow velocity component. Such vibrations are usually caused by pumps and other vibration sources. This vibrational velocity component is particularly undesirable when the fluid flow is completely stopped as by means of a valve. Under these circumstances the vibration of the particles produces a Doppler shifted received signal that will provide an indication of flow at the output of the flowmeter, notwithstanding the fact that the flow has been reduced to zero by a valve. In general, the cumulative reflected vibratory signal from the reflectors in the fluid is substantially less in magnitude than the cumulative signal received from particles that are flowing, even though such flowing particles may include a vibrating component. The reason that the signal from vibrating particles is significantly smaller is that the received signal is due to the sum of the signals from each particle and in the vibrating mode, there is a strong tendency for the signals from the individual particles to cancel each other and to thus provide a signal at any frequency of magnitude significantly smaller than the magnitude from normal flowing particles in which the effect of the reflected signals from the flowing particles is additive.
The receiving channel in ultrasonic Doppler flowmeters generally incorporates a zero-crossing detector as an input element in the frequency-to-voltage converter. In order to prevent the zero-crossing detector from being influenced by noise, it is common practice to have a built-in hysteresis in the zero-crossing detector. With hysteresis the zero-crossing detector indicates a crossing only when the signal exceeds the hysteresis threshold level in either a positive or a negative direction.
In order to avoid a zero-flow signal caused by vibrating reflectors, some ultrasonic Doppler flowmeters are provided with a means to reduce the gain of the receiving channel so that the Doppler signal applied to the zero-crossing detector that is produced by the vibrating reflectors in zero-flow conditions is smaller than the hysteresis threshold level of the zero-crossing detector. To accomplish this, the user is instructed to produce a zero-flow condition and to gradually decrease the gain of the receiving channel until the indication from the flowmeter is zero. While this procedure eliminates the zero-flow error, it does introduce error into the flowmeter signal under normal flow conditions. It can be shown mathematically for any given rms level of a signal applied to a zero-crossing detector with hysteresis-threshold that the average frequency of the output will deviate from the input frequency inversely exponentially as the ratio of the hysteresis-threshold to the rms level of the input signal. Thus, using a gain adjustment of the receiving channel to reduce its gain so that there is a correct zero-flow signal introduces errors in the normal operation of the flowmeter by increasing the ratio.
In order to avoid the problems of flowmeter errors associated with reduction in gain of the receiving channel, Applicants have proposed a circuit arrangement using discriminating means to distinguish between a vibratory motion and a flow motion of the fluid particles. This discriminating means includes a comparator amplifier responsive to the amplified received Doppler signal. The threshold level of the comparator amplifier is adjusted under conditions of zero-flow when the Doppler signal from the reflectors in the fluid is due solely to a vibratory mode to cause the frequency-to-voltage converter to be disabled and produce a zero-flow output signal and to enable the frequency-to-voltage converter when the Doppler signal from the reflectors is caused by fluid flow. Such an arrangement avoids the problem of reducing the receiver gain to avoid zero-flow error. When the fluid begins to flow and the received signal exceeds the threshold level of the comparator amplifier, the ratio of the hysteresis to the rms value of the received signal is small to create a minimal error in the reading of the Doppler flowmeter.
An object of this invention is to remove errors in ultrasonic Doppler flowmeters due to zero-flow vibrating particles without increasing errors during normal operation.